powerpc/mm: Fix _PAGE_PTE breaking swapoff
authorAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Mon, 11 Jan 2016 15:49:34 +0000 (21:19 +0530)
committerMichael Ellerman <mpe@ellerman.id.au>
Tue, 12 Jan 2016 00:02:35 +0000 (11:02 +1100)
commit44734f23de2465c3c0d39e4a16df7735b23fd142
tree9437405c5fff14e2820c0359367386686af80b69
parent68adb7bfd66504e97364651fb7dac3f9c8aa8561
powerpc/mm: Fix _PAGE_PTE breaking swapoff

Core kernel expects swp_entry_t to consist of only swap type and swap
offset. We should not leak pte bits into swp_entry_t. This breaks
swapoff which use the swap type and offset to build a swp_entry_t and
later compare that to the swp_entry_t obtained from linux page table
pte. Leaking pte bits into swp_entry_t breaks that comparison and
results in us looping in try_to_unuse.

The stack trace can be anywhere below try_to_unuse() in mm/swapfile.c,
since swapoff is circling around and around that function, reading from
each used swap block into a page, then trying to find where that page
belongs, looking at every non-file pte of every mm that ever swapped.

Fixes: 6a119eae942c ("powerpc/mm: Add a _PAGE_PTE bit")
Reported-by: Hugh Dickins <hughd@google.com>
Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
arch/powerpc/include/asm/book3s/64/pgtable.h